nixos/test-driver: fix regression when using legacy start command

The aarch64-linux versions of the boot.uefiUsb and boot.uefiCdrom tests
were broken by b0fc9da879.
That commit was a refactor which omitted the qemuBinary option, which was
previously available in the legacy start command. This restores that
option and fixes the tests previously mentioned.

nixos/lib/test-driver/test_driver/machine.py

@@ -241,9 +241,15 @@ class LegacyStartCommand(StartCommand):
         cdrom: Optional[str] = None,
         usb: Optional[str] = None,
         bios: Optional[str] = None,
+        qemuBinary: Optional[str] = None,
         qemuFlags: Optional[str] = None,
     ):
-        self._cmd = "qemu-kvm -m 384"
+        if qemuBinary is not None:
+            self._cmd = qemuBinary
+        else:
+            self._cmd = "qemu-kvm"
+
+        self._cmd += " -m 384"
 
         # networking
         net_backend = "-netdev user,id=net0"
@@ -381,6 +387,7 @@ class Machine:
             cdrom=args.get("cdrom"),
             usb=args.get("usb"),
             bios=args.get("bios"),
+            qemuBinary=args.get("qemuBinary"),
             qemuFlags=args.get("qemuFlags"),
         )